1. Field of the Invention
This invention relates to an optical scanning system for use with an attitude determining system of a vehicle, such as a spacecraft, and more particularly, relates to an optical scanning system effected by a mechanical scanner.
2. Description of the Prior Art
A complete vehicle attitude determining system must determine the vehicle orientation about three axes (usually designated pitch, roll, and yaw axes). For simplicity, the descriptions of the prior art, and of the present invention, will be in terms of single axis systems. Such descriptions will illustrate the salient principles, features, and advantages of the various systems. An extension and application of the principles of a single axis system to a multiple axis system are understood in the art.
Optical star trackers and scanning systems known in the art, make use of three basic methods for utilizing the information derived from the star field of particular application in satellite attitude sensing. By one method, one or more movable optical sensors or trackers are arranged to each lock onto a single identifiable star and a servo mechanism mechanically repositions the tracker to electrically null an error signal and thus to maintain the locked-on condition. The relative angle of the line-of-sight (LOS) of the star from the spacecraft is determined by measuring the angle of the tracker.
The second method is that of scanning the star field with no motion of the optical sensor relative to the spacecraft. As compared to the tracker method, a scanner may be a completely passive device mechanically. If attached to an earth-oriented spacecraft, the sensor scans a swath of the star field at the orbital rate, generating an output series of pulses corresponding to the stars that are sensed by the scanner. Such a scanning system is often termed a "mapper." The pulse pattern derived from the output is compared with a star catalogue and independent star-crossing times are identified. Such information is used to compute the relative angles of the LOS of identified stars.
In prior art systems using the third method, a mechanical scanning motion of the sensor is used, instead of the motion of the spacecraft, to provide the required scanning of the star field. This may be done with a moving reticle, or may utilize a mirror to scan, the mirror being mounted to a rotating wheel on the spacecraft such as the momentum wheel of a dualspun stabilized spacecraft.
A typical prior art device of the third type makes measurements using a telescope-type optical system focused on a slit or reticle in front of a radiation detector. The telescope assembly is rigidly mounted on a rotating platform, such as a momentum wheel of a spacecraft or other vehicle, so that its optical axis scans a plane, or a conical pattern, as the platform rotates. When the focused radiation of a celestial object (such as the sun, stars, or planets) crosses the slit or reticle line, the radiation detector generates an output pulse. If the instantaneous orientation of the platform is measured at the time of the detector output pulse, the LOS of the object, may be determined. The platform orientation measurement must be related to a known zero or reference orientation to the vehicle coordinates. A scanning system using such a device manifests errors due to unavoidable optical and mechanical irregularities and due to timing and threshold irregularities of the optical detector and associated electronic circuits.